Hot Tearing Formation Research Articles

A new hybrid local radial basis function collocation method for 2.5D thermo-mechanical modelling of continuous casting of steel

This study presents a new strong-form meshless method to solve the thermo-mechanical problem of the solidification process in the continuous casting of steel. A two-dimensional slice that travels in the casting direction is modelled in the Lagrangian system. The newly developed mechanical model is one-way coupled to the thermal model, where the heat flux due to the mould, sprays, rolls and radiation are imposed to solve heat transfer in the strand. The resulting temperature and metallostatic pressure govern the Norton-Hoff visco-plastic model used for computing shrinkage of the solid shell and induced residual stresses. The results are used to estimate critical areas susceptible to hot-tearing formation. The mechanical model uses a generalised plane strain assumption that includes linear strains perpendicular to the slice and enables the computation of the straightening of the strand. The thermo-mechanical model is spatially discretised with a local radial basis function collocation method (LRBFCM). The mechanical part includes a new hybrid method that combines LRBFCM with finite differences for increased stability. The presented work shows how the developed model is used to assess the impact of casting velocity on the solid shell shrinkage and the probability of hot-tearing occurrence in the continuous casting of square billets.

A hot tearing propagation model for steel

Hot tearing is a significant challenge in material preparation process, including high-performance alloy casting, large ingot production and additive manufacturing, etc. However, the propagation process of hot tears, especially in steel, is still unclear at present. This study proposed a new model for hot tearing propagation that considers the energy balance, the relationship between the propagating mode and the growth rate of hot tears when they undergo irreversible and rapid propagation. To evaluate this model, a new testing device was developed and the hot tearing behaviors of low and high carbon steels were observed. The tests demonstrated a notable correlation with the model. Furthermore, a new criterion for propagation of hot tearing was put forward. This work provides a more comprehensive physical model and criteria for simulating and predicting the formation of hot tears.

Hot Tear Formation During the Casting of Al–Zn Binary Alloys

The hot tearing susceptibility of Al–Zn binary alloys with solute contents of Al–0.5%Zn, Al–1%Zn, Al–2%Zn, and Al–4%Zn, with all compositions being in wt%, is investigated through physical experiments and numerical simulation. The temperature at the hot spot, the designed critical point for hot tearing, and the load at the end of the test bar are measured and compared with the simulation results. The test samples with 0.5 and 1.0% Zn show hot tears while those with 2.0 and 4.0% Zn have no sign of such tearing. The correlation between solid fraction, strain, stress, and hot tearing indicator is revealed using the simulation results of the Al–1%Zn and Al–4%Zn alloys. Just before solidification, the Al–1%Zn alloy is found to have not only a much higher strain rate at the hot spot of the test bar than that of the Al–4%Zn alloy, but also a higher strain gradient along the longitudinal direction. It is believed that both the high strain rate and high strain gradient are attributable to the formation of hot tears in the Al–1%Zn alloy.

Understanding the roles of interdendritic bridging on hot tearing formation during alloy solidification

Dendritic bridging plays a key role in hot tearing formation and crack propagation, but it has yet to be clearly understood. This article describes the formation process of interdendritic bridging and its effect on the propagation of hot tearing. Two types of interdendritic bridging are proposed: mechanical bridging where dendrites are in contact with each other but are connected only by the surface tension of liquid film and metallurgical bridging where dendrites are metallurgically bonded at their contact points. Mechanical bridging cannot effectively impede crack propagation, thus exhibiting a brittle fracture characteristic, while metallurgical bridging can hinder crack propagation, resulting in a ductile fracture characteristic. Experiments on a high manganese steel were carried out to provide evidence of these two types of bridging and illustrate unique features on fractured surface that are associated with these two types of bridging. In this paper, the process of dendritic bridge formation is clearly described and its effect on hot tearing propagation is discussed.

Revealing hot tear formation dynamics in Al–Cu alloys with X-ray radiography

Hot tears can arise during the late part of alloy solidification because of the shrinkage of isolated liquid as it turns to solid and may have a catastrophic effect on cast tensile properties. Although there are correlations to suggest alloy hot tear sensitivity to casting conditions, they do not capture the influence of microstructure on tearing, such as second-phase particles or intermetallic compounds (IMCs) commonly present in engineering alloys. We use in situ X-ray radiography to quantify the formation and growth behaviour of hot tears in Al-5Cu and Al-5Cu-1Fe alloys during solidification. An automated hot tear detection, tracking and merging algorithm is developed and applied to reveal the role of Fe-rich IMC particles, typical of recycled alloys, on hot tear behaviour. These defects are termed hot tears here on the basis of their complex, extended inter-connected morphology, distinct from more rounded shrinkage porosity. We also visualise and quantify the velocity of interdendritic flow driven by solidification shrinkage, and estimate the pressure changes due to shrinkage. Hot tearing starts at lower solid fraction when IMCs are present due to reduced interdendritic flow, and hot tear formation is more spatially homogeneous, less clustered and more numerous. We show that the largest, most damaging hot tears form from many merging events, that is enhanced by the presence of IMCs.

New Insights into the Features of Hot Tearing Formation in High-Carbon Steel Under Tensile Loading

New Insights into the Features of Hot Tearing Formation in High-Carbon Steel Under Tensile Loading

The cracking behavior of the new Ni-based superalloy GH4151 in the triple melting process

In this paper, the mechanism of crack formation in VIM, ESR, and VAR of GH4151 alloy ingots is discussed. The cracks in VIM ingots are hot tearing, mainly caused by severe segregation of elements such as Nb, Ti, and Al in VIM electrodes, which promotes the precipitation of (γ+γ′) eutectic and Laves phases and thus leads to the formation of hot tearing. On the other hand, cracks in ESR and VAR ingots are cold cracks caused by the uneven distribution of γ′ phases. The temperature range for the precipitation of (γ+γ′) eutectic phases is 1160°C–1148 °C, while that for γ′ phases is 1137°C–1072 °C. At temperatures below 884 °C, the uneven precipitation of γ′ phases results in a maximum internal stress of 1096 MPa at room temperature. Based on computational fluid dynamics (CFD) simulations, to avoid large internal stresses caused by the precipitation of γ′ phase, the ingot should be removed from the water-cooled crystallizer within 57 min. Annealing is an effective measure to improve the quality of electrodes. After annealing treatment, the γ′ phase in the alloy transforms from short rod-shaped to square-shaped, and the decomposition of Laves phase promotes the precipitation of μ phase and σ phase from the matrix, improving the mechanical properties of the alloy.

Hot tear prediction in large sized high alloyed turbine steel parts - experimental based calibration of mechanical data and model validation

The main defects in heavy steel castings are related to hot tear formation during solidification. Depending on the steel grade, design, and local solidification conditions, it is possible to predict regions with higher risk of hot tear formation during the casting process. However, steels containing Boron show more complex crack and defect patterns compared to common steel casting alloys. The mechanisms behind the Boron induced hot tearing is investigated in this work to understand the influence of Boron enrichment during solidification and the influence on hot tearing. The experimental work includes the investigation of phase diagrams and the corresponding fractions of the solid and liquid phases depending on temperature using thermal analysis e.g. DSC and HT-LSCM. The hot tearing sensitivity and mechanical properties during solidification are obtained in the Submerged Split Chill Test, SSCT. In addition IMC-B 3-point bending tests are performed to determine high-temperature material properties in the solid state. The work is part of a research project where the final goal is to improve the hot tear predictions based on experimental work and carry out a benchmark simulation of a real sized casting and use it to show the agreement between the numerical results and extensive non-destructive testing from industrial observations.

Hot tearing behavior of AZ91D magnesium alloy

Hot tearing is a serious destructive solidification defect of magnesium alloys and other casting metals. Quantitative and controllable measurements on the thermal and the mechanical behavior of an alloy during its solidification process are crucial for the understanding of hot tearing formation. We developed a new experimental method and setup to characterize hot tearing behavior via controlled cooling and active loading to force hot hearing formation on cooling at selected fractions of solid. The experimental setup was fully instrumented so that stress, strain, strain rate, and temperature can be measured in-situ while hot tearing was developing. An AZ91D magnesium alloy, which is prone to hot tearing, was used in this study. Results indicate that when hot hearing occurred, the local temperature, critical stress, and cumulative strain were directly affected by strain rate. Depending on the applied strain rate, hot tearing of the AZ91D magnesium alloy could occur in two solidification stages: one in the dendrite solidification stage (fS ∼ 0.81–0.82) and the other in the eutectic solidification stage (fS ∼ 0.99). AZ91D alloy exhibited distinct mechanical behaviors in these two ranges of fraction solid.

Mechanism for the induction and enhancement of inclusions on crack source and simulation analysis for hot tearing tendency of aluminum alloy

Inclusions are typical discontinuous defects, those defects significantly reduce the local mechanical properties of castings during solidification, and play a strong role in inducing and enhancing hot tearing in alloys. Based on the coupling characterization of micro solidification morphology and macro inclusions, this paper proposed a construction method of heterogeneous model containing multi-scale information, and carried out dynamic analysis on the evolution process of stress field at the end of solidification, so as to investigate the influence mechanism of inclusions on the initiation and propagation of hot tearing. The results showed that inclusions and their local content significantly changed the stress field at the end of solidification, which easily resulted in local stress concentration and induced crack source in defects area. During the crack propagation, nearby inclusions greatly reduced the propagation resistance and promoted the formation of hot tearing, which had significant impact on the propagation path of hot tearing. Introducing inclusions defect into stress analysis and comprehensively considering the induction and enhancement of inclusions on hot tearing can better improves the prediction accuracy of hot tearing, which has engineering guiding significance for casting process optimization and product quality control.

Probing interdendritic flow and hot tearing during solidification using real time X-ray imaging and droplet tracking

Despite the well-known importance of controlling shrinkage-induced liquid flow in alloy castings to avoid the formation of catastrophic hot tears during the final stages of solidification, there has been little direct experimental measurement of liquid metal flow and hot tear formation under practical conditions. We use synchrotron X-rays to obtain radiographic video sequences of the solidification of monotectic Al-Pb alloys in which Pb droplets form as a fine-scale emulsion. We track and measure the velocity of thousands of Pb droplets as they move through interdendritic regions due to the effect of liquid to solid shrinkage during the final stages of solidification, up to the point of hot tear formation. Based on the droplet velocities, we present an analysis to estimate the interdendritic liquid velocity as solid fraction increases, and thus the shrinkage pressure drop driving the flow. The analysis is applied for video sequences obtained for both equiaxed and columnar microstructures, each under a range of cooling rates. Our measurements of the critical shrinkage-induced pressure for hot tear formation agree well with prior model-based and theoretical suggestions. The limitations and prospects for droplet tracking measurements of liquid metal flows are discussed.

Investigating Metal Solidification with X-ray Imaging

In the last two decades, X-ray imaging techniques have been used increasingly to study metal solidification in real-time as, thanks to advances in X-ray sources (synchrotron and laboratory-based) and detector technology, images can now be obtained with spatio-temporal resolutions sufficient to record key phenomena and extract quantitative information, primarily relating to crystal growth. This paper presents an overview of the research conducted at the University of Oxford over the last 6 years as a partner in the UK’s Future Liquid Metal Engineering (LiME) Manufacturing Hub. The focus is on in situ X-ray radiography to investigate the solidification of Al alloys, including the formation of primary α-Al crystals, and the formation and growth of secondary intermetallic phases. Technologically, the thrust is to understand how to control as-cast phases, structures and element distributions, particularly elements associated with recycling, as a means to facilitate greater recirculation of aluminium alloys. We first present studies on refinement of primary α-Al, including extrinsic grain refinement using inoculation and intrinsic refinement based on dendrite fragmentation. Second, we describe studies on intermetallic phase formation and growth, because intermetallic fraction, morphology and distribution are frequently a limiting factor of alloy mechanical properties and recyclability. Then we present some of the latest progress in studying liquid flow during solidification and associated hot tear formation. Finally, future research directions are described.

X-ray Imaging of Alloy Solidification: Crystal Formation, Growth, Instability and Defects.

Synchrotron and laboratory-based X-ray imaging techniques have been increasingly used for in situ investigations of alloy solidification and other metal processes. Several reviews have been published in recent years that have focused on the development of in situ X-ray imaging techniques for metal solidification studies. Instead, this work provides a comprehensive review of knowledge provided by in situ X-ray imaging for improved understanding of solidification theories and emerging metal processing technologies. We first review insights related to crystal nucleation and growth mechanisms gained by in situ X-ray imaging, including solute suppressed nucleation theory of -Al and intermetallic compound crystals, dendritic growth of -Al and the twin plane re-entrant growth mechanism of faceted Fe-rich intermetallics. Second, we discuss the contribution of in situ X-ray studies in understanding microstructural instability, including dendrite fragmentation induced by solute-driven, dendrite root re-melting, instability of a planar solid/liquid interface, the cellular-to-dendritic transition and the columnar-to-equiaxed transition. Third, we review investigations of defect formation mechanisms during near-equilibrium solidification, including porosity and hot tear formation, and the associated liquid metal flow. Then, we discuss how X-ray imaging is being applied to the understanding and development of emerging metal processes that operate further from equilibrium, such as additive manufacturing. Finally, the outlook for future research opportunities and challenges is presented.

A Study on Hot Tearing in Direct Chill Casting of Al-Mn-Mg Alloys Using a Multi-scale Approach

A multi-scale approach for simulating hot tearing during the DC casting of aluminum alloys is presented. The novelty of this approach lies in the combination of a macro-scale finite element simulation of the DC casting process with direct prediction of hot tears via a meso-scale multi-physics granular model. This approach is capable of simulating hot tearing initiation, growth, and propagation within a representative volume element of the mushy zone. The change of cooling conditions experienced by the DC cast billet as a result of variations in casting speed as well as non-uniformity of heat extraction from different locations of the billet affect the deformation state, cooling rate, and thermal gradient, which further influence the strain rate, grain size, permeability, and feeding coefficient. Considering all the mentioned parameters, the multi-scale approach emphasizes the fact that hot tearing is a phenomenon resulting from the combination of the tensile deformation and restricted feeding of the mushy zone. The developed hot tearing formation maps identify the locations where hot tearing will occur as predicted by the multi-scale approach for two alloys—AA5182 and AA3104—thus demonstrating the approach’s sensitivity to both processing parameters and alloy composition.

A Critical Conception of Hot-Tearing Susceptibility: How Controlled Diffusion Solidification Enables Shape-Casting with Wrought Aluminum Alloys

Instrumented Constrained Rod Casting (CRC) method and the Controlled Diffusion Solidification (CDS) process were used to study the high susceptibility of the aluminum alloy AA 7068 to hot-tearing. A precise analysis of the measured hot-tearing curves is provided considering the casting design and different feeding mechanisms. Scanning Electron Microscopy was used to study the hot-tear surfaces. It was found that the solidification of the eutectic liquid at the film stage and the phenomenon of solid-feeding strongly affects the hot-tear formation. The early formation of an absorbed eutectic layer on the primary phase due to low rate of back-diffusion is proposed to explain the loss of ductility at the film stage. The layer can serve as an efficient substrate for the eutectic nucleation and growth, hence its formation can advance the eutectic solidification. The isolation of liquid eutectic by its partial solidification results in development of solid-feeding stresses breaking the solid eutectic and provide the required tearing initiators. By increasing the back-diffusion rate, the CDS process avoids formation of the adsorbed solid layer which postpones eutectic solidification and thereby mitigating the hot-tearing susceptibility.

Hot tearing behavior of NZ30K Mg alloy under progressive solidification

Progressive solidification is usually considered an effective strategy to reduce the hot tearing susceptibility of a cast component. In this study, special constrained plate castings with progressive changes in cross-section were designed, which enabled progressive solidification. The hot tearing behavior of a newly developed NZ30K Mg alloy (Mg-3.0Nd-0.2Zn-Zr, wt.%) was studied under progressive solidification using various mold temperature distributions and constraint lengths. Of these, a homogeneous mold temperature distribution is found to be the best option to avoid hot tearing, followed by a local low mold temperature distribution (with a chiller), then a gradient mold temperature distribution. Unexpectedly, compared with the homogeneous mold temperature distribution, adding a chiller does not provide any further reduction in the hot tearing susceptibility of the NZ30K Mg alloy. A high mold temperature and a short constraint length increase the hot tearing resistance of cast Mg alloys. Progressive solidification is not a sufficient and necessary condition to avoid the formation of hot tearing. The two key factors that determine the occurrence of hot tearing under progressive solidification are the maximum cooling rate and the constraint length. Decreasing these values can reduce the incidence of hot tearing.

Qualitative Model Describing Hot Tear Above VIPPO and Numerous Other Design Elements

Over the last couples of years there have been numerous reports of a unique soldering failure resulting in a separation of BGA solder joints from the intermetallic compound at the interposer during reflow. In most cases, the failures were correlated with the use of Via-In-Pad-Plated-Over-Technology. Since the separation could be proven to occur during the phase transition from solid to liquid [1] the term Hot Tear was used. Even though this term is traditionally only used for tears during solidification its scope was extended to tears during any phase transition. Since the Hot Tear results in a very thin separation it is usually not inspectable, neither by means of X-Ray inspection nor by electrical testing, but results in very early field failures.
 In this paper, the general mechanism for the formation of Hot Tears will be discussed and applied to numerous other design elements that can be found on Printed Circuit Board Assemblies. We will show that due to several industry trends e.g. VIPPO, heavy copper PCBs, buried vias, non-eutectic alloys, thinner components, thicker boards, via in pad, etc. the probability of Hot Tears is steadily increasing.

Modelling the Mechanical Attributes (Roughness, Strength, and Hardness) of Al-alloy A356 during Sand Casting.

Sand-casting is a well established primary process for manufacturing various parts of A356 alloy. However, the quality of the casting is adversely affected by the change in the magnitude of the control variables. For instance, a larger magnitude of pouring velocity induces a drop effect and a lower velocity increases the likelihood of cold-shut and mis-run types of defects. Similarly, a high pouring temperature causes the formation of hot tears, whereas a low temperature is a source of premature solidification. Likewise, a higher moisture content yields microcracks (due to gas shrinkages) in the casting and a lower moisture content results in the poor strength of the mold. Therefore, the appropriate selection of control variables is essential to ensure quality manufactured products. The empirical relations could provide valuable guidance in this regard. Additionally, although the casting process was optimized for A356 alloy, it was mostly done for a single response. Therefore, this paper aimed to formulate empirical relations for the contradictory responses, i.e., hardness, ultimate tensile strength and surface roughness, using the response surface methodology. The experimental results were comprehensively analyzed using statistical and scanning electron microscopic analyses. Optimized parameters were proposed and validated to achieve castings with high hardness (84.5 HB) and strength (153.5 MPa) with minimum roughness (5.8 µm).

Meso-scale modelling of semi-solid deformation in aluminum foundry alloys: Effects of feeding and microstructure on hot tearing susceptibility

Abstract In the context of hot tearing susceptibility, the semi-solid constitutive behaviour of two commercially important foundry aluminum alloys at high solid fraction has been simulated. The numerical methodology combines solid mechanics, computational fluid dynamics, and microstructure modelling, i.e a meso-scale multi-physics approach. Feedable and un-feedable domains with different microstructures including equiaxed globular, equiaxed dendritic, and combined dendritic and eutectic were developed. Considering that it is the combined effects of a lack of liquid feeding and limited semi-solid ductility that contribute to hot tearing formation, the effect of feeding and mechanical deformation are studied and discussed. The model demonstrates that the microstructure type and the eutectic formation also have a considerable effect on the liquid channels pressure drop and semi-solid bulk stress and strain, and consequently on hot tearing.

Effect of Yttrium on Hot Tearing Susceptibility of Mg–6Zn–1Cu–0.6Zr Alloys

The effect of Yttrium (Y) addition on hot tearing susceptibility of Mg–6Zn–1Cu–xY–0.6Zr (x = 0, 1, 2, 3) alloys was investigated using a constrained rod casting apparatus which monitored the hot tearing formation with a load sensor and a data acquisition system. The solidification characterization of the investigated alloys was studied through thermal analysis. The thermal analysis results revealed that as-cast Mg–6Zn–1Cu–xY–0.6Zr alloys are composed of primary α-Mg dendrites and several types of second phases, MgZnCu, MgZn2, Mg3YZn6 and/or Mg3Y2Zn3. The hot tearing susceptibility is affected by the addition of Y content that significantly decreases the hot tearing susceptibility of Mg–6Zn–1Cu–xY–0.6Zr alloys due to the grain refinement and high feeding capacity. In addition, the analysis of microstructures and fracture surfaces indicated that liquid film theory and feeding theory are mainly dominant on the hot tearing mechanisms.